The present invention relates to methods of removing refrigerant from a natural gas liquefaction system that uses a mixed refrigerant to liquefy and/or subcool natural gas, and to methods of altering the rate of production of liquefied or subcooled natural gas in which refrigerant is removed from the liquefaction system during shutdown or turn-down of production. The present invention also relates to natural gas liquefaction systems in which the above-mentioned methods can be carried out.
A number of liquefaction systems for liquefying, and optionally subcooling, natural gas are well known in the art. Typically, in such systems natural gas is liquefied, or liquefied and subcooled, by indirect heat exchange with one or more refrigerants. In many such systems a mixed refrigerant is used as the refrigerant or one of the refrigerants. Typically, the mixed refrigerant is circulated in a closed-loop refrigeration circuit, the closed-loop refrigeration circuit including a main heat exchanger through which natural gas is fed to be liquefied and/or subcooled by indirect heat exchange with the circulating mixed refrigerant. Examples of such refrigeration cycles include the single mixed refrigerant (SMR) cycle, propane-precooled mixed refrigerant (C3MR) cycle, dual mixed refrigerant (DMR) cycle and C3MR-Nitrogen hybrid (such as AP-X™) cycle.
During normal (steady state) operation of a such systems the mixed refrigerant circulates inside the closed-loop refrigeration circuit and is not intentionally removed from the circuit. Vaporized, warmed refrigerant exiting the main heat exchanger is typically compressed, cooled, at least partially condensed and then expanded (the closed-loop refrigeration circuit therefore typically including also one or more compressors, coolers and expansion devices) before being returned to the main heat exchanger as cold vaporized or vaporizing refrigerant to provide again cooling duty to the main heat exchanger. Minor amounts of mixed refrigerant may be lost over time, for example as a result of small leakages from the circuit, which may in turn require small amount of make-up refrigerant to be added, but in general no or minimal amounts of refrigerant are removed from or added to the circuit during normal operation.
However, under upset conditions, such as during shut down or turn down of the liquefaction system, mixed refrigerant may have to be removed from the closed-loop refrigeration circuit. During shut down, with the compressors, coolers and main heat exchanger out of operation, the temperature and hence the pressure of the mixed refrigerant inside the closed-loop refrigeration circuit will steadily rise over time as a result of ambient warming of the circuit, which in turn will necessitate removal of refrigerant from the circuit prior to the point at which the build of pressure is likely to lead to damage to the main heat exchanger or any other components of the circuit. During turn-down the inventory of the mixed refrigerant may need to be adjusted to properly match the reduced production rate (more specifically, the reduced amount of cooling duty required in the main heat exchanger) which again necessitates removal of some of the refrigerant from the closed-loop refrigeration circuit.
Refrigerant removed from the closed-loop refrigeration circuit may simply be vented or flared, but often the refrigerant is a valuable commodity, which makes this undersirable. In order to avoid this, another option that has been adopted in the art is to store the refrigerant removed from the closed-loop refrigeration circuit in a storage vessel so that it can be retained and subsequently returned to the closed-loop refrigeration circuit. However, this solution also involves operational difficulties. Mixed refrigerant removed from the closed-loop refrigeration circuit typically will still need to be continuously cooled in order to for it to be stored in an at least partially condensed state, so as to avoid excessive storage pressures and/or volumes. Providing this cooling and condensing duty may involve, in turn, significant power consumption and associated operational costs.
For example, US 2012/167616 A1 discloses a method for operating a system for the liquefaction of gas, comprising a main heat exchanger and associated closed-loop refrigeration circuit. The system further comprises a refrigerant drum connected to the main heat exchanger or forming part of the refrigeration circuit in which refrigerant can be stored during shut down of the liquefaction system, so as to avoid having to vent evaporated refrigerant. The storage drum is provided with heat transfer means (such as for example a heat transfer coil through which a secondary refrigerant is passed) for cooling and liquefying refrigerant contained within the storage drum. The main heat exchanger may also be connected to a supply line through which liquid refrigerant may be injected directly into the main heat exchanger in order to cool down the refrigerant contained therein.
Similarly, IPCOM000215855D, a document on the ip.com database, discloses a method to prevent over-pressurization of a coil-wound heat exchanger during shut down. Vaporized mixed refrigerant is withdrawn from the shell side of the coil-wound heat exchanger and sent to a vessel having a heat transfer coil through which an LNG stream can be pumped, or into which LNG may be directly injected, in order to cool down and condense the mixed refrigerant, which is then returned to the shell side of the coil-wound heat exchanger. In an alternative arrangement, the cooling and condensing of the vaporized mixed refrigerant may take place in the shell side of the coil-wound heat exchanger, by placing the heat transfer coil inside the shell or injecting LNG directly into the shell. The LNG stream can be obtained from a storage tank or from any point in the cold end of the liquefaction unit.
US 2014/075986 A1 describes a method of using the main heat exchanger and closed-loop refrigeration circuit of a liquefaction facility for separating ethane from natural gas during start up of facility, instead of for producing LNG, so as to speed up the production of ethane that is to be used as part of the mixed refrigerant during subsequent normal operation of the liquefaction facility.
US 2011/0036121 A1 describes a method of removing natural gas contaminants that have leaked into a circulating nitrogen refrigerant that is being used in the reverse Brayton cycle for liquefying natural gas. A portion of the nitrogen refrigerant is withdrawn from the cycle, liquefied in the cold end of the main heat exchanger and introduced into the top of a distillation column as reflux. The purified nitrogen vapor withdrawn from the top of the distillation column to returned to the cycle. The liquid withdrawn from the bottom of the distillation column, comprising the natural gas contaminants, may be added to the LNG stream produced by the liquefaction system.
US 2008/0115530 A1 describes a method of removing contaminants from a refrigerant stream employed in a closed-loop refrigeration cycle of an LNG facility. The refrigerant stream may be a methane refrigerant or an ethane refrigerant employed in a cascade cycle, with the contaminant comprising a heavier refrigerant (e.g. ethane or propane, respectively) that has leaked into the refrigerant from a separate closed-loop circuit of the cascade cycle. The system employs a distillation column to remove the contaminants. The contaminated refrigerant is introduced into the distillation column at an intermediate location. A vapor stream of contaminant-depleted refrigerant is withdrawn from the top of the column and returned to its closed-loop refrigeration circuit. A contaminant-enriched liquid is withdrawn from the bottom of the column and discarded.